A phase-locked loop (PLL) is often used to generate clock signals. As well known in the art, a PLL is an electronic circuit that oscillates at a desired frequency and automatically adjusts the phase of its oscillation to match (lock on) that of an input signal. The PLL includes an oscillator, which is often a voltage-controlled oscillator (VCO). A VCO typically includes inverters configured in a ring. An odd number of inverters are needed to oscillate.
In addition to the VCO, a conventional PLL may include a phase/frequency detector to convert a phase difference to a pulse-width-modulated (PWM) error signal, a charge pump and lag lead loop filter to convert the phase error to a control voltage and a buffer amplifier that converts the high impedance control voltage (or tuning voltage) to a low impedance drive for a VCO tune line. The VCO converts the control voltage to an output phase. A programmable frequency divider at the VCO output feeds the divided-down VCO phase back to the phase detector input to complete the loop.
In general, a PLL operates by converting a pulsed error voltage to a DC voltage that is used to control the VCO. This DC control voltage causes the VCO to slightly change the output signal of the VCO in a direction that reduces the phase difference and also the frequency difference between the input and output signals. This error correcting cycle is repeated for each reference frequency cycle period, ultimately resulting in a minimum difference between the two frequencies. The phase-locked loop is described as “locked” when the phase difference between the two signals has stabilized.
PLLs are often manufactured in silicon. As processing nodes for silicon manufacturing continue to get smaller, the power supply voltage available to the PLLs is reduced and the associated noise levels are increased. At the same time, however, many applications employing PLLs (e.g., analog-to-digital converters), are requiring an increased signal-to-noise ratio and operating frequency range. To achieve these requirements, improved oscillators with a broader operating range and reduced noise are needed in the art.